Super Eariy Age Performance And Cracking Analysis Of Steam Curing Concrete

The steam curing concrete technology is advanced in high early strength,high proficiency of mould and high production efficiency,which has been widely used in many giant key infrastructure projects and achieved excellent economic benefits.However,steamed concrete also has many drawbacks.For example,the steaming process will accelerate hydration of cementitious material and variatioin of temperature and humidity in the concrete,which may leads to an increasing tendency in the risk of shrinkage and cracking of concrete Hence,To this end,this paper considers the effects of fly ash and mineral powder,and prepares four sets of proportioned concrete and corresponding mortar and mortar tspecimens The mechanical properties,temperature and humidity transfer and internal stress development of concrete under different curing conditions(natural curing,steam curing,dry heating curing)were studied,and cracking risk was also evaluated.The main results are as follows(1)The deformation of cement-based materials under tensile load under different conditions was studied by the digital image correlation technology(DIC).The tensile stress-strain curve was measured to obtain the tensile strength and elastic modulus of cement-based materials at the early age.Based on the tensile strength and elastic modulus of cement paste and mortar,the tensile strength and elastic modulus of steam curing concrete at early age were deduced.The chemical titration and thermal analysis methods were used to quantitatively study the variation of content of calcium hydroxide(CH)in cement-based materials under different cementitious materials and different curing conditions,and to analyze the influence of evaporation on hydration of cement-based materials at early age.The results show that the ultimate tensile strength and tensile elastic modulus of concrete under steam curing condition develop the fastest,and under dry heating curing condition the next,and under natural curing condition the slowest.The relationship between calcium hydroxide content in different curing methods is:steam curing>dry heating curing>natural curing,which is consistent with macroscopic mechanical results.Replace cement with a single fly ash or complex mineral powder and fly ash,which will reduce the early elastic modulus of cement-based materials.The early elastic modulus growth of concrete specimens with cement mixed with fly ash or mineral powder and fly ash is reduced.Moreover,the early activity of mineral powder at high temperature is slightly higher than that of fly ash.Under the same curing conditions,the content of the mixture with large amount of mineral powder is higher than that of the mixture with large amount of fly ash(2)The temperature and humidity sensors were used to study the change of medium temperature and humidity inside the concrete during the steaming process,meanwhile,the development of internal stress was also calculated.The cylindrical shrinkage test method was used to study the free shrinkage of steamed concrete.The results show that the deformation of steamed concrete mainly occurs during the heating period.With the constant temperature condition,the deformation trend becomes gentle while some shrinkage occurs during the cooling stage.The early thermal expansion coefficient of steamed concrete is not constant The mixture of fly ash and mineral powder will reduce the early thermal expansion coefficient of steamed concrete.The study shows that the early thermal expansion coefficient of steamed concrete is roughly 15-20×10-6/?.The surface of steamed concrete is more susceptible to damage than the interior,and the highest temperature stress on the surface is as high as 1.547MPa.After adding 25%fly ash,the surface temperature stress of concrete decreased by 37.2%,indicating that the incorporation of fly ash can inhibit the temperature stress inside the steamed concrete.The steaming method increases the early free deformation of the concrete,but reduces the late deformation value.Both fly ash and mineral powder reduce the early free deformation of concrete.When the specimen was steamed for 8 hours,the specimen mixed with 25%fly ash was reduced by 3.3%,and the test piece mixed with 20%fly ash and 30%mineral powder was reduced by 20%.Fly ash and mineral powder increased the late free deformation of concrete.When the specimen was cured to 72d,the test piece incorporating 25%fly ash increased by about 17%,and the test piece mixed with 20%fly ash and 30%mineral powder increased by about 65.1%(3)According to the ring shrinkage test,the development of internal restraint stress of concrete under steaming conditions was studied.The cracking performance of steamed concrete was evaluated by comparing the development of ultimate tensile stress and restraint stress.The test results showed that the concrete ring did not crack under steam curing and natural curing conditions.The reason was that the confined shrinkage stress of the concrete specimen under the steam curing method during the steaming period(0-8h)was remarkable,but the corresponding early ultimate tensile stress developed rapidly.Because of ft>?t,the test pieces did no crack.Based on the test results,a prediction model for steaming concrete cracking risk is established:?=?(t)/ft(t)=E(t)(?S+?T)/f(t),when ?>1,the steamed concrete cracks;when 0.7??<1,the steamed concrete has a certain risk of cracking;when ?<0.7,the steamed concrete will not crack.